1) Field of the Invention
The present invention relates to the solar energy devices, and, in particular, relates to a trough solar collector that can be used to collect solar energy for power generation.
2) Description of Related Art
In a conventional trough-type solar power generation system 10, as illustrated in FIG. 1, a plurality of parabolic collector mirrors 20 are used to reflect sunlight toward receivers 18 in which a heat transfer fluid is circulated. The fluid, which can be organic or synthetic oil, is circulated by a pump 12 through a fluid circuit 14 that includes the receivers 18 and a plurality of connection pipes 16. After being heated by the collector mirrors 20, the fluid is delivered to a steam generator 22, in which thermal energy is exchanged from the heat transfer fluid to water in a separate fluid circuit 24. Thus, the heat transfer fluid is cooled in the steam generator 22 and can then be re-circulated to the receivers 18 for reheating. Small storage tanks (not shown) may also be included to store the heat transfer fluid. The water heated in the steam generator 22 forms steam that is circulated to a turbine generator 26, i.e., a turbine 28 coupled to an electrical generator 30. The steam expands and rotates the turbine 28 and the generator 30, thus producing electricity. The steam can be passed through a condenser 32 that, in conjunction with a cooling tower 34, condenses the steam to form hot water that is preheated through preheater 33 and can be circulated back to the steam generator 22 by a pump 36 for re-use.
The parabolic collector mirrors 20, shown in FIGS. 2 and 3, typically can be pivoted such that each mirror 20 can be rotated according to a relative position of the sun 50. For example, a motor and drive 40 can rotate the mirrors 20 about the receivers 18. Thus, each mirror 20 can typically be turned “on” by rotating the mirror 20 about the receiver 18 so that the mirror 20 is directed normal to the sun 50 to collect and reflect solar radiation, as shown in FIG. 3. If not directed substantially normal to the sun 50, the mirror is in an “off” position. Due to the parabolic shape, the mirrors 20 do not heat the receivers 18 unless rotated to the on position, where the receivers 18 are in the focal line of the mirrors 20, and a small deviation in rotational position from the on position results in little or no heating of the receivers 18.
At times of relative darkness, such as at nighttime and during cloudy or otherwise overcast weather conditions, sunlight cannot be used for heating the fluid. The circulation of the fluid is typically continued to avoid problems associated with shutdown and/or startup of the system 10. In particular, circulation can be continued to prevent thermal stresses due to temperature mismatch associated with introducing the fluid into the system 10 at a temperature that differs significantly from the temperature of the receivers 18, and other components of the system 10, as can occur during startup. However, if the fluid is continuously circulated when the system 10 is receiving little or no sunlight, the fluid loses thermal energy to the cooler ambient environment. Further, if the temperature of the fluid falls below its freezing point, the fluid will solidify in the receivers 18 and/or the pipes 16. Uniform thawing of the fluid in the fluid circuit 14 can be difficult, and the expansion of the fluid associated with freezing and/or thawing can over-stress, plastically deform, and eventually burst or otherwise destroy the receivers 18, and other components of the system 10. Therefore, in order to maintain the proper temperature of the fluid, the fluid is heated by electric or gas heaters as the fluid circulates around circuit 14, increasing the energy required for operating the system 10, reducing the overall efficiency, and adding pollutants to the environment.
As noted above, the collector mirrors 20 of the trough-type system 10 are configured either in an “on” position in which solar energy is reflected at a maximum rate to the receivers 18 or in an “off” position in which substantially no solar energy is reflected to the receivers 18. Therefore, if the receivers 18 are emptied at the end of each day and refilled at the start of each subsequent day, it can be difficult using the system 20 to match the temperature of the fluid and the receivers 18 during the daily filling operation.
Thus, there exists a need for a solar energy device that can accommodate and heat different types of heat transfer fluids, including those with varied ranges of operating temperature. Preferably, the device should be capable of preheating and/or thawing the heat transfer fluid so that the fluid does not need to be circulated and/or heated continuously during periods of limited sunlight.